Radiation Detection Utilizing Optical Bleaching

ABSTRACT

A method and device for improving the optical performance (such as time resolution) of scintillation detectors using the optical bleaching technique are disclosed. Light of a selected wavelength is emitted by a light source into a scintillator. The wavelength is selected to meet the minimum energy requirement for releasing of charge carriers captured by the charge carrier traps in the scintillation material. Trap-mediated scintillation components are thus reduced by optical bleaching and the optical performance of the scintillator crystal and the detector is enhanced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional ApplicationsSer. No. 61/709,565, filed Oct. 4, 2012, which provisional applicationis incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to radiation detectors employing scintillatormaterials and particularly to scintillation detectors with improvedoptical characteristics due to reduced charge carrier trapping. Certainarrangements also relate to specific components and configurations ofsuch scintillation detectors and method of making the same.

BACKGROUND

Scintillator materials, which emit light pulses in response to impingingradiation, finds a wide range of applications, including medicalimaging, particle physics and geological exploration. In a typicalscintillation detector for radiation detection, a scintillator crystalis coupled to light sensor, such as a photomultiplier tube (PMT),photodiode, or silicon photomultiplier. The performance of suchdetectors can be adversely affected by various defects in thescintillator crystal. Efforts have been made to improve the scintillatorgrowth process to reduce the concentrations of the various defects andotherwise improve the performance of scintillator materials anddetectors. Nonetheless, there is a continued need for scintillatormaterials and detectors with improved optical characteristics.

SUMMARY

This disclosure relates to a method of improving the optical performance(such as time resolution) of scintillation detectors using the opticalbleaching technique. Certain defects, commonly known as “charge traps”or “charge carrier traps,” degrade optical performance parameters suchas rise time and decay time. Optical bleaching allows the release ofcharge carriers captured by the traps in the material by irradiation ofthis material with light, thereby reducing trap-mediated scintillationcomponents and enhancing the optical performance of the scintillatorcrystal and the detector.

In one example, a light source, either active (e.g., diodes) or passive(e.g., a phosphor or co-dopant) is disposed adjacent, or otherwise inoptical communication with, a scintillator material. The light source isadapted to emit light of certain selected wavelength of wave lengthsinto the scintillator material. The wavelength or wavelengths of lightemitted into the scintillator material are selected to depopulate thecharge traps, thereby improve the optical performance of thescintillator material.

In another example, a radiation detector includes a scintillator and alight source adapted to emit light of a predetermined wavelength intothe scintillator having charge carrier traps, the predeterminedwavelength being suitable to depopulate the traps.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically depicts a scintillation detector according to anaspect of the disclosure.

FIG. 2 schematically depicts a scintillation detector according toanother aspect of the disclosure.

DESCRIPTION

Many scintillation and optical properties of scintillator materials arestrongly affected by a variety of defects created in thecrystallographic structure during material synthesis and/or as a resultof exposure to high flux of ionizing radiation. These defects, commonlyknown as “charge traps” or “charge carrier traps,” may significantlyalter kinetics of scintillation mechanism, changing the rise time anddecay time of a scintillation pulse. Charge traps may capture largeamount of electrons and holes generated by ionizing radiation in thevolume of scintillator. These charge carriers are temporary excludedfrom the scintillator process or completely lost due to thenon-radiative processes that take place in the material.

This effect was demonstrated by A. J. Wojtowicz et al. ([1] A. J.Wojtowicz, J. Glodo, W. Drozdowski, K. R. Przegietka, “Electron trapsand scintillation mechanism in YAlO₃:Ce and LuAlO₃:Ce scintillators,”Journal of Luminescence, Volume: 79, (1998) p. 275; [2]J. Glodo, A. J.Wojtowicz, “Charge traps and emission kinetics in LuAP:Ce,” Proc. SPIE4412, (2001), 216-220) in the case of well-known scintillators LuAP(LuAlO3) and YAP (YAlO3). The authors of those papers discuss the effectof charge carrier trapping phenomena on the scintillation time profilemeasured at different temperatures. The theory is based on a modelassuming the presence of two distinct mechanisms of energy transferbetween a crystal matrix (crystal lattice) and luminescence centers. Thefirst mechanism is called a “direct” mechanism. It is independent of thesecond, which is called a “trap-mediated” mechanism.

The direct mechanism assumes that the charge carriers (electrons andholes) generated in the crystal as a result of absorption of ionizingradiation are transferred to the luminescence center without interactionwith traps. The second, “trap-mediated” mechanism assumes that chargecarriers are trapped and then released by defects prior to reachingluminescence centers. This latter process may cause some delay in theenergy transfer to the luminescence center.

The direct process results in the emission of the luminescence centerswith a decay time constant equal to the radiative lifetime of thosecenters in the crystal. The “trap-mediated” results in creation ofdelayed components in the scintillation pulse. The values of thesedelays depend on the trap life time (τ_(trap)), energy depth of selectedtrap (E), a frequency factor (s), and temperature (T) as described bythe formula below:

$\begin{matrix}{\tau_{trap} = {S^{- 1}{\exp ( \frac{E}{\kappa \; T} )}}} & (1)\end{matrix}$

where κ is the Boltzmann constant.

Contributions of these two mechanisms to the scintillation time profileare described by the following equation:

$\begin{matrix}{{I(t)} = {I_{0}\lbrack {{\frac{a}{\tau_{rad} - \tau_{0}}\{ {{\exp ( {- \frac{t}{\tau_{rad}}} )} - {\exp ( {- \frac{t}{\tau_{0}}} )}} \}} + {\frac{b}{\tau_{rad} - \tau_{trap}}\{ {{\exp ( {- \frac{t}{\tau_{rad}}} )} - {\exp ( \frac{t}{\tau_{trap}} )}} \}}} \rbrack}} & (2)\end{matrix}$

where I(t) is the intensity of the light at the time t, I₀ is totalmeasured light in the single pulse, a and b are branching coefficientsthat describe the normalized fractions of the total scintillation lightemitted in one pulse due to direct- and trap-mediated mechanismsrespectively, τ₀ denotes the rise time, and τ_(rad) denotes radiativedecay time of luminescence center, where τ₀<<τ_(rad) (see reference [1]above for details).

The first term in Equation (2) corresponds to direct component, and thesecond term to delayed one. It is noted that in the second term in theequation above, the rise time equals to the shorter of the two timeconstants τ_(rad) and τ_(trap) while the decay time equals to the longerone.

Examining the equations above, one readily sees that for certaintemperatures T and selected trap parameters E and s, the τ_(trap) can beshorter than τ_(rad), and then a trap-mediated component in thescintillation profile will contribute to the rise time of the pulse. Onthe other hand if τ_(trap) is longer than τ_(rad) it will contribute tothe decay time of the scintillation pulse. In either case, the rise anddecay time will have an effect on timing resolution of the radiationdetectors utilizing scintillator containing traps. Therefore, it wouldbe very desirable to reduce the contribution trap-mediated components inscintillation process to improve the performance of the radiationdetector.

One way of improving the performance of the scintillators would be tosynthesize a defect-free material. However, this goal is very difficultto achieve. Another method would be to release captured charge carriersfrom defects using thermal depopulation of traps. However, an increaseof temperature is not always practically achievable in radiationdetector designs due to the numerous problems, including thermalquenching of light, degradation of components of the detector, as wellas instability of its main detector characteristics.

One aspect of the present disclosure is a method for improving anoptical performance parameter, such as timing resolution, of radiationdetectors by reduction of trap-mediated scintillation components usingoptical bleaching technique (or optical trap depopulation).

Optical bleaching allows the release of captured charge carriers in thematerial by irradiation of this material with light. By carefullyselecting light wavelengths, the population of captured carriers oncertain trap levels (terms a and b in the equation) can be decreased soas to reduce the intensity of the trap-mediated components in thescintillation pulse. Optical bleaching energy required to initiate theprocess is very often higher than equivalent thermal energy for the same‘probability of escape’ of trapped carriers. (see, e.g., [3] S. W. S.Mckeever, “Thermoluminescence of Solids,” Cambridge University Press(1995), p. 137.)

For example, W. Drozdowski et al. ([4] W. Drozdowski, P. Dorenbos, R.Drozdowska, A. J. J. Bos, N. R. J. Poolton, M. Tonelli, M. Alshourbagy,“Effect of Electron Traps on Scintillation of Praseodymium ActivatedLu₃A₅lO₁₂,” IEEE Trans. Nucl. Sci. Volume: 56, No. 1, (2009) p. 320-327)demonstrated the effect of optical bleaching on the intensity of X-rayluminescence measured at wide range of temperatures (10-300K). Theauthors used infrared laser for optical bleaching. Under exposure tolight, the intensity of X-ray luminescence as a function of temperaturewas very stable and was not affected by traps. However, when lightsource was set off the X-ray luminescence intensity was dropping due tothe charge carrier trapping effects.

According to another aspect of the disclosure, a radiation detectorincludes a scintillator material (such as a single-crystaloxyorthosilicate or halide scintillator), and a built-in light source.The light source, which can be either active (powered) or passive(unpowered) irradiates the scintillator material during normal operatingmode of the detector.

Referring to FIG. 1, in one aspect of the disclosure, a radiationdetector (100) includes a scintillator (110) made of a scintillatormaterial and a light source (120) adapted and disposed to emit light ofcertain selected wavelengths into the scintillator. The scintillator(110) is adapted to receive radiation, which enters the scintillator (asdepicted by the arrow) and interacts (130) with the scintillationmaterial. The detector (100) further includes one or more lightdetectors (150 a, 150 b), such as photomultiplier tubes (PMTs),photodiodes, or silicon photomultipliers, which are disposed to receivethe light generated by the scintillator (110), optionally through anoptical filter (140, described below).

In the example device shown in FIG. 1, the light source (120), which maybe a dispersed light source, is disposed on the radiation-receivingsurface of the scintillator. Alternatively, as in the detector (200)illustrated in the example shown in FIG. 2, the light source (220) canalso be disposed on the outside perimeter of the scintillator (110). Ina further example, the light source can be dispersed throughout thescintillator, as in the case (described further below) of a scintillatorco-doped for the purpose of optical bleaching.

In one aspect of this disclosure, one or more semiconductor diodes orlaser diodes, with specifically chosen wavelengths of emission, can beused for targeting selected traps. These wavelengths of emission arechosen to meet requirements for the minimum energy needed for opticalbleaching process to occur. Optical elements can be built as arrays ofdifferent diodes emitting light of wavelengths targeting differenttraps. The activation energy for these traps is dependent on the energydepths of the trap levels and their frequency factors.

In another aspect of this disclosure, passive optical bleaching lightsources, which do not require electrical power to operate, can be used.For example tritium radiation sources with selected phosphor materialscan be used to generate the desired wavelength of light. Tritiumradiation sources can be encapsulated in phosphor materials to generatethe desired wavelength(s) of light. The ionizing radiation emitted byTritium source is converted to desirable wavelength by the phosphormaterial.

Using multiple wavelengths of light (multiple light sources), traps ofmultiple energy depths can be selectively targeted. This approach willallow one to eliminate selected trap components that are directlyresponsible for the changes in the scintillation decay time and risetime, or to reduce the effect of traps that influence scintillationlight output.

In another example, phosphor materials surrounding the scintillatorcrystal media can be used. In this passive configuration the lightgenerated by scintillator can be partially converted to light of thewavelength necessary to release captured charge carriers by thephosphor. If layer of phosphor has sufficient stopping power to convertdirectly ionizing radiation the light needed for optical bleaching canbe generated by a detected radiation signal without absorption ofscintillator light.

In another aspect of the present disclosure, a passive optical bleachingis achieved by specifically selected co-doping schemes during synthesisof scintillator material. The additionally added co-dopants generatesthe desired optical bleaching light during the scintillation processitself. In this case some small portion of the energy absorbed in thescintillation process is used to generate light of the desiredwavelength(s) to optically depopulate traps. As an example, some RareEarth ions such as Er or Yb can be used as a co-dopants depends on theselected scintillator matrix.

The light used for optical bleaching scintillators may interfere withscintillation light detected by PMTs or optical sensors. In this case,this light can be filtered out by one or more optical filters (140). Inone example, an interference filter (140) with a transmission windowoptimized for scintillator light can be used. The filter (140) canperform the role of a light guide for detectors that require a lightsharing to determine the position of the interaction.

In operation, the light source (120, 220) irradiates the scintillatormaterial (110) during normal operation of the detector (100, 200). Theoptical bleaching allows charge carriers to be continuously removed fromtraps and used in the scintillation process without any delays.

Thus, scintillators and scintillation detectors with improved opticalproperties, including improved time resolution, can be made with opticalbleaching. Because many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention, the inventionresides in the claims hereinafter appended.

1. A radiation detector, comprising: a scintillator comprising ascintillation material having charge carrier traps capable of capturingcharge carriers generated in the scintillation material by radiation, alight source optically coupled to the scintillator and adapted togenerate and transmit into the scintillator light of a wavelengthappropriate to fulfill a minimum energy requirement necessary to releasecaptured charge carriers.
 2. A method of radiation detection, comprisingdetecting radiation using a scintillator to generate a light signal uponreceiving radiation; detecting the light signal generated by thescintillator; and emitting a light of a predetermined wavelength in tothe scintillator at the same time as the detecting step.
 3. Theradiation detector of claim 1, wherein the light source comprises apassive light source.
 4. The radiation detector of claim 3, wherein thepassive light source comprises a phosphor material.
 5. The radiationdetector of claim 4, wherein the passive light source further comprisesa radioactive material disposed to emit radiation to the phosphormaterial to cause the phosphor material to generate light of thewavelength.
 6. The radiation detector of claim 4, wherein the phosphormaterial at least partially surround the scintillation material and isdisposed to receive the radiation and generate light of the wavelengthupon receiving the radiation.
 7. The radiation detector of claim 3,wherein the passive light source comprises a co-dopant in thescintillation material, the co-dopant adapted to generate the light ofthe wavelength when the scintillation material receives the radiation.8. The radiation detector of claim 7, wherein the co-dopant comprises Eror Yb.
 9. The radiation detector of claim 1, wherein the light sourcecomprises a diode.
 10. The radiation detector of claim 9, wherein thelight source comprise an array of diodes.
 11. The radiation detector ofclaim 1, further comprising a light detector optically coupled to thescintillation material and adapted to detect light generated by thescintillator material upon receiving radiation.
 12. The radiationdetector of claim 11, further comprising a filter disposed between thescintillation material and light detector and adapted to substantiallyblock the light from the light source from entering the light detector.13. The radiation detector claim 1, wherein the light source comprises aplurality of light sources each adapted to generate light of arespective wavelength, the wavelengths of light generated by theplurality of light sources being different from each other, each of thewavelengths being appropriate to fulfill a respective minimum energyrequirement necessary to release charge carriers from charge carriertraps of a respective energy depth.
 14. The method of claim 2, whereinemitting a light of a predetermined wavelength comprises opticallycoupling a light source to the scintillator, wherein the wavelengthsatisfies the minimum energy requirement for releasing of chargecarriers captured by charge carrier traps in the scintillator.
 15. Themethod of claim 14, wherein detecting the light signal generated by thescintillator comprises optically coupling the scintillator to a lightdetector.
 16. The method of claim 15, further comprising filtering lightfrom the scintillator to substantially block the light from the lightsource from entering the light detector.
 17. The method of claim 2,wherein emitting a light of a predetermined wavelength comprisesemitting light from a semiconductor diode.
 18. The method of claim 2,wherein emitting a light of a predetermined wavelength comprisesemitting light from a phosphor.
 19. The method of claim 8, whereinemitting a light of a predetermined wavelength comprises irradiating thephosphor from a radioactive source.
 20. The method of claim 2, whereinemitting a light of a predetermined wavelength comprises including inthe scintillator a co-dopant adapted to emit light, by a scintillationprocess, of wavelength satisfying the minimum energy requirement forreleasing of charge carriers captured by charge carrier traps in thescintillator.